Venus reaches its greatest illuminated extent as the evening “star” in just a few days, on February 17, 2017. Greatest illuminated extent means the planet’s daytime side, or illuminated side, is covering more square area of Earth’s sky than at any other time during this current evening apparition of Venus. And it means that Venus is brighter around now than at any other time during its approximate 9.6-month reign in the evening sky. Plus Mars is still near Venus in the west after sunset. Amazing evening sky! Watch for these two worlds.
The time of greatest illuminated extent for Venus – for Earth as a whole – will be 7:00 UTC. Converting Universal Time to what our clock time in U.S. time zones, Venus is at its greatest illuminated extent on February 17, at 2:00 a.m. EST, 1:00 a.m. CST, 12:00 midnight MST and at 11:00 p.m. PST (on February 16). But no matter where you live on Earth, Venus appears in the western evening sky after sunset.
Although Venus always shines as the third-brightest celestial body, after the sun and moon, it’ll be shining at its brightest best in the evening sky for the next several days.
Why is Venus so bright now? You might think Venus appears most brilliant when we see its disk as most fully illuminated from Earth. Not so. If you were to observe Venus with the telescope at its greatest illuminated extent, you’d see that Venus’s disk is only a touch more than one-quarter illuminated by sunshine.
A full Venus is always on the far side of the sun from us, so its disk size at full phase is always small. It’s only when we see Venus as a crescent that this world comes close enough to us to exhibit its greatest illuminated extent, at which time its daytime side covers the greatest area of sky.
Venus entered the evening sky on June 6, 2016 and will leave it on March 25, 2017. Where will it go? It’ll pass more or less between us and the sun on March 25. Astronomers call that an inferior conjunction of Venus.
Take a look at the chart below. Venus transitioned from the morning to evening sky when Venus swung directly behind the sun as viewed from Earth (superior conjunction) on June 6, 2016. Then Venus reached its greatest eastern (evening) elongation from the sun on January 12, 2017, and will reach its greatest western (morning) elongation from the sun on June 3, 2017. As a rule of thumb, it takes about one year for Venus to move from superior conjunction to its greatest western elongation.
Bird’s-eye view of Earth’s and Venus’ orbits
Midway between Venus’ greatest evening elongation (February 17, 2017) and greatest morning elongation (June 3, 2017), Venus will swing in between the Earth and sun (at inferior conjunction) on March 25.
However, Venus is not going to pass precisely between us and the sun at this upcoming inferior conjunction on March 25, 2017. If it were, Venus would transit the sun, as it did in June of 2012. In fact, Venus will swing 8o north of the sun, so sky gazers in the Northern Hemisphere might be able to view Venus in both the evening and morning sky for a few to several days around the time of Venus inferior conjunction on March 25, 2017!
Because Venus orbits the sun inside Earth’s orbit, we can never see Venus opposite (180o) the sun in our sky (like the full moon). We can’t even see Venus 90o from the sun (like the half-lit quarter moon). At most, Venus strays no farther than 47o from the sun in our sky. This is called Venus’ greatest eastern elongation when Venus appears in the evening sky and greatest western elongation when she predominates over the morning sky.
Venus reaches its greatest elongation in the evening sky about 72 days before inferior conjunction, and then reaches its greatest elongation in the morning sky some 72 days after inferior conjunction. If you look at Venus through a telescope at these times, you’ll see that its disk is about 50% illuminated by sunshine.
Venus exhibits its greatest illuminated extent about 36 days before – and after – inferior conjunction. Through the telescope, Venus appears about 25% illuminated in sunshine at these times. Thirty-six days before inferior conjunction, it’s Venus’ brightest appearance in the evening sky; thirty-six days after inferior conjunction, it’s Venus brightest appearance in the morning sky.
Let the golden triangle help you to remember these Venus’ milestones. The two base angles equal 72o and the apex angle equals 36o. Quite by coincidence, Venus’ greatest elongations happen 72 days before and after inferior conjunction, and Venus’ greatest illuminated extent happens 36 days before and after inferior conjunction. (See above diagram of Venus’ and Earth’s orbits.)
Bottom line: Enjoy Venus at dusk and early evening in mid-February 2017. Even though this world is only about one-quarter illuminated in sunshine right now, as seen from Earth, Venus is nonetheless shining at its brightest best in the evening sky! Plus, Mars is nearby.